The elastomeric copolymer of ethylene (E) and hexafluoropropylene (HFP) is excellent in heat resistance, oil resistance and chemical resistance (particularly alkali resistance), just as other fluorine resin and elastomers are.
Because the copolymerizability of ethylene and hexafluoropropylene is extremely low and the copolymerization reaction does not proceed in moderate polymerization conditions, copolymerization has been conducted under severe polymerization conditions of high pressure or using radiation in the past.
For example, in J. Polym. Sci. A, 12, page 627 (1974), copolymerization is conducted in a high pressure of 4.9 MPaG (50 kgf/cm2G) by irradiating with radiation at ambient temperature. However, using radiation requires special facilities and is not suitable in terms of safety.
The method of copolymerizing ethylene and HFP at a high temperature and high pressure (650 kg/cm2 to 2,000 kg/cm2, 115 to 200° C.) batchwise or continuously is disclosed in the Journal of the Chemical Society of Japan, 1980, (1), p 112 to 120. However, though the vulcanization properties of the obtained copolymer are excellent, the polymerization method is not emulsion polymerization. In addition, as mentioned before, because the reaction conditions are extremely harsh, there is the problem of an increase in the building costs of the production facilities.
U.S. Pat. No. 4,039,595 describes conducting copolymerization at a low temperature of 30° C. at 3.76 MPaG (545 psiG) using a polymerization initiator such as diisopropyl peroxydicarbonate. However, the polymerization method is suspension polymerization and the obtained copolymer is a low molecular weight substance without elastomeric properties.
WO 94/24175 pamphlet describes copolymerizing ethylene, HFP and tetrafluoroethylene (TFE) at 50° C. under high pressure of 3.43 MPaG (35 kgf/cm2G) using ammonium persulfate as a polymerization initiator. However, the polymerization rate is low and the obtained copolymer has many branches and is inferior in processability.
Furthermore, as a method for preparing a fluorine-containing elastomer by copolymerizing at a low temperature, JP-A-56-163105 describes redox polymerization using water soluble persulfate and a reducing agent comprising hydroxymethane sulfinate and ethylenediaminetetraacetic acid or a salt thereof. However, there are no descriptions mentioning E-HFP copolymers or the increase in vulcanization properties by lowering the polymerization temperature.
The present invention aims to provide a process for preparing an elastomeric E-HFP copolymer with few branches, large molecular weight and excellent vulcanization properties.